The detection of the motion characteristics of particles is frequently conducted for characterizing the physical properties of various particles such as metal, polymer, and ceramic particles, and so on. The detection of the motion characteristics of particles in a flow channel is also used as an important means for characterizing biological cells. As an example, many researchers have analyzed the flow of red blood cells, white blood cells, and sperms in microfluidic chips, thus characterizing the oxidative states, chemotaxis properties, and motilities of the biological cells, respectively. [the red blood cell characterization: M. Zborowski, et al., “Red blood cell magnetophoresis, Biophys. J., Vol. 84 (2003) pp. 2638-2645, the white blood cell characterization: D. Irimia, et al., “Microfuidic system for measuring neutrophil migratory responses to fast switches of chemical gradients, Lab. Chip., Vol. 6 (2006) pp. 191-198, and the sperms characterization: B. S. Cho, et al., “Pssively driven integrated microfluidic system for separation of motile sperm, Anal. Chem., Vol. 75 (2003) pp. 1671-1675.]
Further, since the detection of the motion characteristics of particles in a flow channel allows the flow characteristics, such as the flow rate and velocity profile of a fluid, containing particles, in a flow channel, it is a very important technology for measuring a flow rate, diagnosing the internal state of the flow channel, and visualizing flow [flow visualization: D. Sinton, Microscale Flow Visualization, Microfluid Nanofluid, Vol. 1, No. 1 (2004) pp. 2-21.]. In particular, in microfluidic devices, a system, which is sufficiently cheap to be integrated into a single chip and makes it simple to detect the motion characteristics of particles in the flow channel, is urgently required.
Methods of detecting the motion characteristics of particles in a flow channel which are frequently used these days, include particle image velocimetry, fluorescent image tracking velocimetry, laser scattering and dispersion analysis, electrical impedance analysis (particle impedance analysis), etc.
Particle image velocimetry and fluorescent image tracking velocimetry are methods of acquiring the flow image of a typical particle or a fluorescent particle having a fluorescent material attached thereto in a flow channel, analyzing the image, and detecting the motion characteristics of the particle in the flow channel, and are disclosed in U.S. Pat. Nos. 5,124,071 and 5,333,044, respectively. These method are advantageous in that they can analyze the motion of particles having a complicated flow in the flow channel, but are disadvantageous in that, since a microscope, an imaging device and a computer for image data processing are required, high costs are incurred, and since the structure of the system is complicated, it is difficult to implement the microfluidic devices. Further, the method using fluorescent particles is also disadvantageous in that it additionally requires a procedure of attaching fluorescent materials to particles.
The laser scattering and dispersion analysis is a method of radiating laser light onto a flow channel through which particles flow, measuring the degree of optical scattering and dispersion of the laser, appearing when the particles pass through the flow channel, and detecting the motion characteristics of particles in the flow channel, and is disclosed in U.S. Pat. No. 6,867,410. This method does not require a procedure for separately attaching fluorescent materials, but is disadvantageous in that a laser light source, a plurality of optical sensors, and complicated optical instruments are required, so that the system is expensive and has a complicated structure, and thus it is difficult to implement the microfluidic devices. Further, the method has limitations in that particles in a flow channel must have optical characteristics that enable the efficient scattering and dispersion of laser light. Moreover, all of the above optical methods also have limitations in that, since they use optical principles, the material used to make the flow channel must be transparent in order to minimize optical loss.
The particle impedance analysis is a method of measuring the electrical impedance occurring when particles pass through a measurement region of an electrode, and of counting the particles in a flow channel or measuring the size of the particles, the location of the flow of the particles, and the flow rate of the particles, and is disclosed in U.S. Pat. Publication No. 2003-0102854. The method, proposed in U.S. Pat. Publication No. 2003-0102854, is implemented in order to determine the relative location of the flow of the particles in the flow channel on the basis of the fact that the magnitude of the electrical signal varies according to the location of the particles, which pass through a region between electrodes forming a pair, relative to the electrodes. This method is advantageous in that, since an electrical measurement method is used, optical equipment is not required, and since the system thereof has a simple structure, the system can be implemented in the microfluidic devices. However, the measurement of the particles is limited to the measurement of a relative location, rather than an absolute location. The magnitude of the electrical signal is also influenced by the size of the particles, variation in the impedance (spectral impedance) of particles according to measurement frequency, and the conductivity of a fluid containing the particles, as well as the location of the particles between electrodes. Accordingly, in order to precisely measure the location of the flow of the particles in the flow channel, preliminary information, such as the size of the particles, spectral impedance of the particles, and the conductivity of the fluid containing the particles, must be known. Further, this method is disadvantageous in that, when the conductivity of the fluid containing the particles is dynamic, when particles have large differences in size and spectral impedance despite being the same type of particles, or when different particles having various sizes and various spectral impedances are mixed together, it is difficult to measure the motion characteristics of the particles.